The eye has an amazing ability to see detail and perceive contrast in both very bright and very dim objects. In order to appreciate the brightness range that the eye can utilize, it is appropriate to look at the range of brightness that the eye may normally encounter. First it is essential to establish a unit of measure for light. Units like watts or watts per square meter would make sense. But traditionally, light was measured in terms of a standard candle. This leads to a set of measures that are special to light.

The basic measure of illuminance is called a candela. This is the total visible light emitted in all directions by a standard candle. The standard candle is an actual candle made of a certain mixture of waxes and of a certain size. Another measure of illuminance is the lumen. One candela is equal to one lumen per steradian. Since there are 4*pi  steradians in a complete sphere, a lumen is a candela divided by 4*pi. (about 12.57).  Note that these two units are measures of the power of the source and the power through a steradian around the source.

Extended objects need another measure of their brightness. This is the power per unit area and it is called luminance. The units are candelas per square meter or lumens per square meter. Thus these two words are thus both measures of power (brightness) but one is the power of the source, illuminance. The other the power of  or through a surface of a given area, luminance. (most unfortunate that these words are so similar)

To get some idea about the luminance caused by a stellar object inspect the following table. The stellar magnitude is the usual brightness rating given to objects by astronomers. Each change in magnitude of 1 unit is a factor of  the 1/5 root of 100. Thus 5 magnitudes is 100 times and 10 magnitudes is 10 E4 times. The table gives the luminance or the power through a surface in terms of lumens per square meter. This number is important since it tell how much light energy is intercepted by a telescope of a given aperture.
 

Dark Adaptation

The physiology of dark adaptation dark is complex. The phenomenon is highly dependent upon the individual viewer. So, as with all biological effects, only average behavior can be specified. In its simplest form, it is a fact that a viewers ability to perceive light changes and gets better if the eye is allowed to remain in the dark for some time.  This is a chemical effect in the retinal of the eye. (too complex to describe here)   Never-the-less everyone experiences this effect. Typically the change in sensitivity is from 2 to 6 magnitudes after 20 to 30 minutes of darkness. It may vary greatly from person to person for reasons related to physical condition of the eye. Typical variations for persons with otherwise normal sight are about 2 magnitudes.

These numbers mean that the sensitivity of the eye may increase after 30 minutes by as much as 250 times (6 magnitudes). That is a large improvement. Brief exposure to bright lights wipe out this improvement almost immediately. Thus viewers should shield their eyes from any light while viewing and especially from very actinic light. Dim red light is the least damaging but even that causes some decrease in acuity. Adaptation also depends on the size of a spot of light shown on the retina in a complex way. The best advice is to severely limit exposure of the retina to any light to retain maximum brightness acuity.

These numbers and variations from person to person show why some viewers claim to see Mag 8 stars regularly while other have trouble seeing  Mag 4 stars under similar conditions. People's brightness acuity simply varies by a great deal and may depend significantly on the use of tobacco, alcohol and other chemicals. On a broad average, most persons can see Mag 6 stars on a clear dark night.
 

The Structure of The Retina of The Eye

The structure of eye is complex, here are outlined only a few factors that directly affect astronomical viewing. The very center of viewing, that is, the point in space that attracts our direct attention is focused on a region of the eye called the fovea centralis. This portion of the eye, only a few degrees in size, is crammed with visual cones.  These cones have the ability to see color but are not highly sensitive to brightness. Immediately surrounding the fovea centralis is a large ring of receptors called rods.   The rods have little sensitivity to color but are quite sensitive to brightness. They see in black and white. There are of course some cones mixed in with the rods so color is perceived everywhere but only when the excitation is sufficiently bright. The rods are about 4 magnitudes more sensitive to light than the cones.

There is a spot about 15 to 18 degrees to the nasal side of the retina where the optic nerve enters the eye and is attached to the retina.  This spot is blind and may be a couple of degrees in diameter. Notice that since the spot is to the nasal side, the blind region on the surface being observed is in the temporal direction because the lens of the eye turns the image upside down and left to right. But it is important to recognize that when viewing objects they should not be viewed is such a way as to place them on the blind spot.

On the other hand, to the temporal side of the retina, especially at 15 to 20 degrees,  there are an abundance of cones. This makes the region 15 to 20 degrees to the temporal side of the retina very sensitive to brightness.   Thus astronomers use what is called averted vision. By forcing the eye to concentrate attention just a bit in the temporal direction, the object is moved onto the region of the eye with the greatest brightness sensitivity.   As one eye moves the object into the region of greater sensitivity the other eye moves the object into the blind spot. But viewing is generally do with one eye and whatever eye is used, moving the center of attention toward the temporal side does the desired function.

It is also necessary, when using averted vision to hold the object on the sensitive spot for some time to get the full effect of averted vision.  A period of 4 to 7 seconds is usually optimal.
Thus, is required concentration and practice to use averted vision techniques successfully. However, it is worth while to practice this technique since the increase in brightness sensitivity is considerable.
 

Sensitivity and Contrast

In order to see very faint stars and some detail in those enticing gray smudges in the sky, it is necessary not only to have the best possible brightness sensitivity but also an image size that optimizes contrast within the faint objects. This topic is even more obscure and strange if you will than the above facts. There are really two things that must be realized that are physiological facts and will have to go without proof.  One is that the eye can discern contrast better when the scene is bright and much more poorly when the scene is dim. The other is that the ability of the eye to discern contrast differences increases with the size of the objects placed in conjunction to each other. These two factors work together in a complex way when trying to see contrast within dim objects.

The first effect is quite obvious. As the skies get darker, we see more and more, fainter and fainter stars. With the telescope we see even more stars since the object is greatly magnified thus spreading out the background light and still gathering the star, which is a point source, into a relatively good point image. When looking at a diffuse object with a telescope, both the background and the object are spread out in a similar way so the actual contrast between them does not change.  Still, most astronomers claim to see, and in fact do see, increasing contrast as the magnification is increased within a certain range. This fact is caused by the second property of the eye. Namely, the eye can discern contrast better if the objects viewed are larger.

These facts and psychophysical phenomena combine to hint that there might be an optimum magnification factor for viewing extended objects through a given telescope. All even slightly experienced viewers will say "this is certainly true."  It is partially the reason for having so many eyepieces for various magnifications and fields of view. (the other is the innate desire for having more toys of course)   So it is in fact not only good but necessary to have this array of viewing tools.

What is happening is that there is a struggle between the eyes ability to see contrast, which gets poorer with decreasing brightness and the eyes improving ability to see contrast as the image gets larger.   There should be and in fact is an optimum field of view and magnification for every object and for each persons eyes. Now it is possible to take much data available in the literature on the eye and make some estimates of the optimum magnification for an object with a given surface brightness.  The most brilliant of these analyses is given in a fine book by Roger N Clark "Visual Astronomy of the Deep Sky,"  Sky Publishing Corporation and Cambridge University Press,  1990.   It is however rather difficult to follow all of the details.

One could go through a lengthily analysis of limiting magnitudes for various telescopes, various sky conditions, various dark adaptation and even the effect of filters on the ability to see objects and detail in them. Elaborate formulas are however not the object of this discussion.  Anything that brightens the image and increases the contrast helps to see detail and separate the object from the background. This includes, larger diameter telescopes which gather more light, more perfect optics and various filters which darken light pollution and still allow the spectral lines of the objects being observed through.

Thus here we will only make a few observations on the most promising practice to follow for most amateurs. The contrast detectable by the eye improves by about a factor of ten when the size of the object at the eye goes from 1 to 10 arc minutes. After that, the improved contrast detectability seems to stabilize. This depends to some extent on the brightness but that is a secondary factor. Thus, a useful viewing tactic is this. Start with low magnification. The object is probably too small for the eye to detect detail easily even though it is bright. Move to a higher magnification. Then detail becomes easier to detect and the general view of the object improves. Continue to increase magnification. Finally as the magnification becomes too great, the brightness gets too low and the eye is no longer able to detect detail as well.

At this point, I refer you to the book mentioned above. It gives a great deal of information about the issues mentioned above with many charts and calculations. In addition there is an extended discussion of viewing of all  the Messier objects one by one with both images and drawings. The discussion give a realistic evaluation of what might be seen with an 8 inch telescope and at what magnification the object is best viewed. Details about the objects size and surface brightness are also given so one can estimate its visibility with a larger telescope. There is also a list of over 600 objects and the optimum magnification to use for a variety of telescope sizes.

Unfortunately this book, printed first in 1990 is now out of print and hard to find. It is very well worth hunting for.

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